The Soviet Union had just failed in Afghanistan they were in no shape and had no taste for another intervention.
They still had enough strength, and our dear "allies" especially East Germans were more than eager to take part in the invasion.
there were plenty of Red Army troops stationed in Poland
60 thousand, compared to Polish half-a-million plus reservists. Also, the Russian troops in the barracks were "kindly asked" by nearby Polish units' commanders to stay there (and they did!). I know it from old army officers, who remember also very well what was happening in the Baltic Sea and along the borders in 1980-81.
there were no plans for the Soviets to intervene in Poland
Of course there were. Read the book, Iron. As I said, it's well researched and documented.